One of communication protocols suitably used for a wiring in an automotive vehicle is a SbW (Safe by Wire) method. The SbW method is such that electric power supply and communication are only performed with two lines in order to reduce the number of wirings.
FIG. 14 shows a construction that one master 1 (e.g., an ECU for the vehicle) and multiple slaves 2a-2f are connected in a line with a pair of buses 3a, 3b. Each slave 2a-2f includes a switch 4 therein. When a certain slave 2a-2f has trouble, the switch 4 at the certain slave 2a-2f separates off the bus 3a so that only normal slaves 2a-2f continue to communicate among the slaves 2a-2f and the master 1.
FIG. 15 shows a waveform of bus voltage in the bus 3 when the master 1 and/or the slaves 2a-2f drive the bus 3 so that a data is transmitted by the SbW method. In this SbW method, firstly, the master 1 drives the bus 3a, 3b with a voltage level of VLP in a power supply period (i.e., power phase) so that electricity for operating each slave 2a-2f is supplied to the slave 2a-2f. A data period (i.e., data phase) is performed after the power phase. In the data phase, the master 1 and/or the slaves 2a-2f transmit a one-bit data. A total period of the data phase and the power phase is defined as a one-bit period (i.e., a 100% bit phase). The length of the 100% bit phase defines a communication speed. In some cases, the communication speed of one communication system may be changed dynamically.
When the master 1 transmits the data during the data phase, the master 1 drives the bus 3a, 3b with a voltage level of VL0 or VL1. These voltage levels correspond to the data of “0” and “1”, respectively. When the slave 2a-2f transmits the data during the data phase, the master 1 drives the bus 3a,3b with a voltage level of VL0. Here, the length of the power phase is equal to the length of the data phase.
At this time, if the slave 2a-2f does not drive the bus 3a, 3b, the voltage level maintains at VL0. Accordingly, in this case, the data of “0” is transmitted. If the slave 2a-2f drives the bus 3a, 3b with the voltage level of VL1, the data of “1” is transmitted. Here, the voltage level of VL1 is lower than the voltage level of VL0. If the slave 2a-2f drives the bus 3a, 3b with the voltage level of VLS1, an interruption is generated in relation to the master 1. Here, the voltage level of VLS0 is lower than the voltage level of VL1.
FIGS. 16A and 16B show one example of data communication between the master 1 and the slave 2a-2f. The master 1 drives the bus 3a, 3b with voltage levels of VLP and VL0 in a 200% bit period, which is twice longer than the one-bit period in the communication rate at that moment, so that a SOF (i.e., start of frame) signal is transmitted. The SOF signal shows start of communication. The slave 2a-2f recognizes the start of communication when the SOF signal is transmitted to the bus 3a, 3b. 
Then, the master 1 transmits two data-bit signals of MSA and SEL so that the master 1 specifies a communication mode. After that, the slave 2a-2f transmits the data. Specifically, in FIG. 16A, a data of “SLOT 1_DATA” and a following data of “CRC” represent a data transmitting period of the slave 2a. The data of CRC (i.e., cyclic redundancy check) is a signal for detecting an error, and attached to the data of SLOT 1_DATA. Similarly, the slave 2b transmits a data of “SLOT 2_DATA” and a data of “CRC,” the slave 2n transmits a data of “SLOT N_DATA” and a data of “CRC,” and so on. In an idling period for waiting the communication, in which the communication is not performed, the master 1 repeats to drive the bus 3a, 3b with the voltage level of VLP and the voltage level of VL0 alternately.
Thus, in the SbW method, the master 1 supplies electricity through the pair of buses 3a, 3b in the power supply period, and further, the master 1 communicates with the slaves 2a-2f. Thus, the SbW method provides a serial communication system.
When the above communication system is used for an air bag system of the vehicle, multiple slaves 2a-2f correspond to acceleration sensors, which are mounted on various parts of the vehicle. When one of sensors detects impact of accident, a detected signal is transmitted from the one of sensors as a slave 2a-2f to the master 1. The master 1 outputs an ignition instruction signal to an inflator of the air bag system. Then, the inflator generates gas so that the air bag expands.
In the above SbW method, if the system functions, the master 1 continues to supply electricity to the slaves 2a-2f. Accordingly, even in the idling period, the master 1 drives the bus 3. Thus, the communication signal having a predetermined frequency may be continuously generated, so that a higher harmonic wave of a fundamental harmonic wave having the predetermined frequency increases with its power. Thus, a noise may be generated easily.
In view of the above problem, a spectrum diffusion communication method and a Bluetooth technique together with a frequency hopping method are provided. Specifically, in these methods, a communication frequency is changed so that a power spectrum is diffused. Thus, a peak intensity of a noise is reduced.
For example, a system inputs a PWM (i.e., pulse width modulation) signal into a driving circuit for driving a load such as a motor so that the system controls to switch on/off. In this system, when a duty ratio of the PWM signal is constant, a higher harmonic wave of the PWM signal increases with its power, so that a noise may be generated easily.
Here, since the SbW method does not spread sufficiently, the inventors could not find an appropriate prior art at this moment.
In the above technique such as the spectrum diffusion communication method, a noise level may be not sufficiently reduced in some cases. Thus, it is required for a communication system to reduce a noise sufficiently. Further, it is required to provide a method for determining a duty ratio of PWM control.